The neck of a bottle is given high heat resistance and rigidity by whitening the neck in the thermal crystallization treatment. Such a neck is used for the synthetic resin bottle to be filled with tea, fruit juice, liquid seasoning, retort-packed food and the like at a high temperature or to be subjected to an intermediate process, such as a thermal sterilization process, and is used especially for the biaxially drawn, blow-molded bottle made of a polyethylene terephthalate resin (hereinafter referred to as PET bottle). (See Patent Document 1.)    [Patent Document 1]: Application No. 1998-058527
A multi-threaded screw structure is adopted to shortcut the rotating movement required to fit or remove the screw cap or to reduce the amount of synthetic resin material required to mold a neck. The multi-threaded screw structure is utilized, especially for most of wide-mouth bottles, so that the neck does not become too tall.
In most cases, the cap screwed on the PET bottle is not an aluminum pilfer-proof cap that requires large screwing strength, but a synthetic resin pilfer-proof cap that does not require much strength. The neck of a PET bottle can have the wall thickness and bead ring size that are smaller than those used in the case where the aluminum pilfer-proof cap is put on. Thus, the synthetic resin pilfer-proof cap gives a higher resource saving effect.
As shown in the rolled-out view of FIG. 11, the neck in the conventional art is provided with a number of screw threads 30, which amount to three in FIG. 8, and these threads 30 are disposed at an equal central angle on the outer surface of round neck wall 2. Each main thread 40 primarily carries out the screwing function of each screw thread 30. The anterior half of the main thread 40 of a screw thread 30 is located above the posterior half of a neighboring screw thread 30. In turn, the posterior half of the main thread 40 of the first screw thread 30 is located beneath the anterior half of the main thread 40 of still another screw thread 30.
Each screw thread 30 is provided with a starting portion 50 at the start of the main thread 40 and an ending portion 60 at the end of the main thread 40. This starting portion 50 has reduced dimensions for smooth screw engagement and release from the mold. The ending portion 60 too has reduced dimensions for smooth release from the mold.
The main thread 40 of a screw thread 30 is located above the main thread 40 of another screw thread 30 in a main zone that performs the screwing function. Between a main zone and another neighboring main zone there is an auxiliary zone that helps the main zones perform the screwing function. This auxiliary zone includes a starting portion 50 of a screw thread 30, an ending portion 60 of another screw thread 30, and a part of the main thread 40 of still another screw thread 30.
The screw engagement between the neck and a screw cap is achieved in the main zones, which are equally spaced in the circumferential direction and in which the above-described screwing function is performed. On the other hand, the above-described auxiliary zones do not need to accept large screwing and fitting force. Therefore, the starting portion 50 and the ending portion 60 of these screw threads 30 are molded in sizes as small as possible within a range in which mold release and screw engagement can be achieved smoothly.